WASHINGTON -- Sen. Barack Obama, desperate to cut down front-running Sen. Hillary Clinton, did not take advantage of one opening in Thursday night's Las Vegas Democratic presidential debate. Obama pulled his punches on Clinton's September vote for a resolution that he earlier said can be used to go to war against Iran. His reticence may be traced to his co-sponsorship of a similar hawkish amendment back in March.

Obama was softer toward Clinton than last month when he called her "reckless" for voting to name the Iranian Revolutionary Guard as a terrorist organization, claiming it would give President Bush a pretext to attack Iran. For her part, Clinton did not raise Obama's inconsistency and was uncharacteristically silent about Iran. The two leaders for the Democratic nomination were muzzled by mutually assured destruction, reflecting a Democratic dilemma.

Democrats want to assume a strong anti-terrorist position while deploring U.S. military action against Iran as it develops nuclear weapons. While such an attack before Bush leaves office is reviled on the Left, no Democrat can be seen as soft on an Iranian Islamist regime whose nominal president denies the Holocaust and calls for the destruction of Israel. The trick is to condemn both Dick Cheney and Mahmoud Ahmadinejad.

This balancing act was upset Oct. 11 when the Manchester (N.H.) Union Leader published Obama's op-ed column assailing Clinton's vote for the resolution sponsored by Republican Sen. Jon Kyl and Independent Democratic Sen. Joseph Lieberman. By designating the Iranian Revolutionary Guard as a terrorist organization, wrote Obama, "we're still foolishly rattling our sabers" in passing "this reckless amendment," 76 to 22. Obama contended "the Bush Administration could use the language in Lieberman-Kyl to justify an attack on Iran as part of the ongoing war in Iraq." Obama missed the vote.

Obama energized Lanny Davis, Washington lawyer and longtime supporter of the Clintons. In an Oct. 16 letter to The New York Times, Davis noted Obama was one of 68 senators -- including Clinton -- who on March 22 co-sponsored Senate Resolution 970, using language similar to Lieberman-Kyl in branding the Iranian Revolutionary Guards. On his Oct. 24 website, Davis wrote: "It is a complete mystery why Sen. Obama or his campaign managers thought he could get away with criticizing Sen. Clinton on the Kyl-Lieberman resolution and calling it reckless while knowing about his own co-sponsorship of S. 970."

The response to Davis came one day later from another Washington lawyer: Gregory Craig, Davis's comrade defending President Bill Clinton and now Obama's foreign policy adviser. E-mailing supporters, Craig did not address S. 970 but indicated the Kyl-Lieberman resolution went beyond a terrorist designation. Calling co-sponsors Kyl and Lieberman "two of the most hawkish members of the Senate on Iran," Craig suggested their resolution "can be used to justify a U.S. attack on Iran."

This dispute was not raised by Obama in Las Vegas as part of his many-sided attack on Clinton. Not until the floor was opened to "undecided" voters did an Iraq war veteran's mother ask about Iran. Sen. Joseph Biden, chairman of the Senate Foreign Relations Committee, said of Kyl-Lieberman: "It's a serious, serious mistake. . . . [I]t convinced the rest of the Muslim world this is really a war against Islam. . . . If he [Bush] takes the country to war in Iraq [he meant Iran] without a vote of Congress . . . then he should be impeached."

CNN's Wolf Blitzer, the moderator, immediately turned to Clinton as "the only one on the stage who did vote for that resolution." Biden, perhaps remembering Bill Clinton's complaint that "the boys" were ganging up on his wife, interjected: "I wasn't attacking Sen. Clinton." Obama next called the resolution "a mistake," but he said of Clinton only this: "I agree with Hillary that we've got to initiate bold diplomacy." Clinton said not a word about Iran and kept away from Obama's past desire to brand the Revolutionary Guards.

So, Iran got few minutes at Las Vegas, with important questions unanswered. Could Clinton or Obama co-exist with a nuclear Iran? Do they forswear the military option in Iran? Would they join Biden in impeaching Bush if he attacks Iran? They cannot want to face those difficult queries.